The 305-d lactation and 60-d dry period were adopted during World War II in the United Kingdom and later in the U.S. (Knight, 1998). This management strategy provided the best balance for optimizing milk yield and genetic progress. Today more than 70%of U.S. dairies are still managing for a 60-d dry period (NAHMS, 1996). However, several changes have occurred since the adoption of the 60-d dry period such as: improved milk production of 5,000 kg per lactation per cow, greater emphasis on profit, use of artificial insemination and embryo transfer, total mixed rations (TMR), increased milking frequency, altered photoperiod, and bST. Despite these changes, research on a dry period requirement has received little attention.
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